Bump libextobjc to 0.4 -- fixes iOS7/64-bit breakage, and more.

I had a temp version of this bump here -- https://github.com/CocoaPods/Specs/pull/7766 -- which was rightfully declined. On request upstream happily tagged and bumped this version.

* 0.4 fixes breakage on iOS7/64-bit.
* It also removes experimental features. Upstream moved all experimental features to an 'experimental' branch, so people who need those features aren't totally SOL.